Found this in another site
"To build a "legal" semi-only version, the receiver must not be able to accept an original F/A bolt...
There are (2) ways to accomplish this. One, you can use an original ID receiver and then install a blocking rail or two, use a reduced ID receiver.
Extreme caution must be used however if you plan to build from an original ID receiver to avoid possibly finding yourself in violation of the law.
1. Do not have a "parts kit" and the 80% receiver at the same location until AFTER the receiver has been modified with a blocking rail.
2. Do not cut ANY openings on the receiver until after you have installed the blocking rail. (keep in mind that the "receiver" is considered the actual firearm, it you complete the cutouts prior to installing the rail, you WILL be in possession of a MG...)
Below are some "basic" rules that will apply to any semi-conversion of a MG or SMG.
If you are not licensed to build "post dealer samples" and are considering purchasing an original ID receiver for a "semi-only" build, there are many things you will need to take into consideration.
**Note: If building from a "reduced ID" receiver, items #1, #2, & #3 will not be required as the reduced ID accomplishes the same thing.**
1. You can not have this receiver (even in it's 80% state) and a parts kit in your possession at the same time. To do this could be considered "constructive intent".
2. Use of this receiver to build a "semi-only" will require that the receiver have a "blocking rail" installed to prevent the installation and function of a "Full Auto" bolt.
3. You will need to install the blocking rail "before" you make any of the other cuts. Failure to do this could put you into possession of a NFA regulated F/A firearm. (the receiver is considered the firearm).
4. Once the blocking rail is installed into the receiver, you will need to make the following modifications to your original bolt. You must do these items in this order...
A. Grind off the "fixed" firing pin.
B. Grind off the "lower" feed lips from the face of the bolt.
C. Drill the bolt to accept a "floating" firing pin.
D. Grind or Mill the bolt for use with a conventional hammer/disconnector assembly.
5. You can now complete the cuts on the receiver. You will need to determine exactly how you will want to install your FCG (fire control group) and what US made parts to use to count as compliance parts.
6. You will also need to "permanently" extend the length of the barrel to a min. of 16" with either a welded extension or a 16" replacement barrel.
This information is offered solely as a guideline to help you comply with BATF regulations, as I understand them. Any building of a firearm must meet all NFA/BATF regulations and it is the responsibility of the builder to insure that they are followed".
http://www.weaponeer.net/forum/forum_posts.asp?TID=522&PN=1&TPN=4